Stellar photometry with the Carte du Ciel plates - I. Method

We present a method for measuring the magnitude of stars on the photographic plates of the Carte du Ciel. There are between 95 and 162 non-variable stars of known B and V magnitudes on each plate, which covers an area of 2 square degrees on the sky. The densities of these stars, measured with a commercial scanner, are used to calibrate the density-to-intensity conversion relation of each plate. The derived magnitudes decrease linearly away from the center and then brighten again more steeply toward the edge, presumably because of vigneting and coma. A distance- and density-dependent correction for these two effects is determined for each plate; this is necessary if one wants to determine the light curve of a given star with plates belonging to adjacent fields. We find a magnitude-dependent color effect, which is not understood, and thus not corrected for. The final accuracy of the calibration depends on the distance from the center of the plate, and is optimal near the center. The method is tested on five known variable stars.